Video signals, for example, those providing picture display information on a television set, comprise a series of frames that are displayed at a suitable frame rate to permit persistence of vision. The frame rate may also represent the time interval that exists between successive frames. Video frames which are in the analog domain may be represented as a continuous or time varying signal. However, in the digital domain these frames may be formatted as a series of digital images.
In the United States for example, the standard power cycle for residential usage is 60 Hertz (Hz). This low bandwidth presented a problem for the displaying of video signals. Accordingly, the earliest television sets were only capable of displaying video frames at a rate of 30 frames per second (fps) due to this limited of bandwidth. In this regard, one video frame may be displayed every one-thirtieth ( 1/30th) of a second approximately. Although 30 frames per second may be adequate for maintaining persistence with normal vision, when viewed on a television set, a flickering effect may be perceived. Flickering may be defined as an unintended rapid change in the instantaneous intensity of a video display. Flickering may occur whenever an instantaneous intensity of a video display decreases before the video display has been updated with new information required to maintain screen intensity. A refresh rate or rate at which a video display may be updated may be increased to maintain the screen intensity, thereby reducing flickering.
A display technique called interlacing was subsequently developed to address the flickering effect, enhance the effective frame rate and better utilize available bandwidth of a video signal. Interlacing divides a frame into one or more sequential fields, which may be selectively displayed. A parameter called an interlace ratio may be used to describe a manner or sequence in which video raster lines may be scanned to represent a video frame. For example, a 1:1 interlace ratio may signify a field consisting of adjacent vertical lines. Essentially, interlacing provides a technique, which may increase the frame rate without increasing the available bandwidth.
The national television standards committee of the USA (NTSC) has defined a standard which utilizes an interlace ratio of 2:1. The NTSC standard utilizes a technique in which two fields are scanned per frame. Using an interlace ratio of 2:1, a frame may be divided into an odd or upper field and an even or lower field. Accordingly, this may significantly reduce the effects of flickering by displaying each of the upper and lower field every one-sixtieth ( 1/60th) of a second. In this regard, there refresh rate is increased from one-thirtieth ( 1/30th) of a second to one-sixtieth ( 1/60th) of a second and the bandwidth remains the same.
FIG. 1a is an exemplary diagram 100 illustrating an interlaced scanning technique. Referring to FIG. 1a, there is shown an upper field 102 and a lower field 104 of an interlaced video for a display having 10 lines. The interlaced video of FIG. 1 utilizes an interlace ratio of 2:1. In the upper field 102, the odd lines 1, 3, 5, 7, 9, 11, 13 and 15 are depicted. In the lower field 104, the even lines 2, 4, 6, 8, and 10 are illustrated. The current line being scanned is line 10 and the next line to be scanned is line 12. Lines 12, 14 and 16 have not yet been scanned. Lines 1–16 may be scanned from left (L) to right (R). The upper field 102 and the lower field 104, when combined, form the entire video frame. In order to minimize the effects of flickering, adjacent lines in the lower field 104 may need to be scanned at a rate that permits the lines in the lower field 104 to be updated before corresponding adjacent lines in the upper field 104 have begun to fade. For example, line 2 should be scanned before line 1 has begun to fade and line 4 should be scanned before line 3 has begun to fade.
FIG. 1b is an exemplary diagram 120 illustrating a non-interlaced scanning technique. Referring to FIG. 1b, there is shown a display frame 122 being scanned in a progressive or non-interlaced format. In non-interlaced scanning, all the lines for the entire display frame 122 are scanned in one pass. In this regard, the lines 1–13 of the display frame 122 may be successively scanned from left (L) to right (R). Line 13 of the display frame 122 is the current line being scanned. Lines 14, 15 and 16 have not yet been scanned.
As personal computers (PCs) and video monitors became popular, non-interlaced scanning began competing with interlace scanning. The competition was due to several factors such as the fact that luminescent materials used to make the display terminals was less prone to fading and video processing circuits and/or processors were capable of handling much higher refresh rates. The advent of standards and related technologies such as high-definition television (HDTV) and motion picture expert group (MPEG) video have also increased the popularity of progressive scanning. Additionally, the convergence of various communications technologies and the need to provide data on demand accessible by a single communication device have further led the various other scanning formats, along with a greater embedded base in interlaced and progressive scanning technologies. Accordingly, conversion techniques are required to convert between various scanning formats utilized by communication devices
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.